Power meters are an increasingly crowded market these days, but that didn’t stop Specialized from jumping in with a complete line of its own power meters, called Power Cranks. The systems will be sold both aftermarket and included stock on select Specialized bikes. The company claims that Power Cranks are both the lightest and most accurate carbon crank arm power meters on the market. Is that true? The answer is, for a piece of hardware that’s empirically testable, surprisingly hard to nail down.

What’s New for 2018?

This is a brand-new product line for Specialized. The company has never made power meters before, and has limited experience with cycling computers. The hardware and firmware is supplied by existing power meter maker 4iiii, with some of the hardware and firmware (like the housings) created specifically for Specialized. However Specialized created its own software application (available in iOS and Android) for setup, configuration and zero offset functions. The power meter communicates over ANT+ and Bluetooth wireless protocols, so it should be compatible with any of today’s computer head units.

Specialized is making a big commitment to power meters, with five offerings starting with a version based on Shimano's new Dura-Ace crankarms. Photograph courtesy of Specialized

Multiple Options, Multiple Prices

Specialized will offer five different configurations of the system, at prices from $425 to $1,500 for single- and double-sided power meters. That’s aggressive pricing for all but the most-expensive model, when compared to competitors like Stages and 4iiii’s own line. Here’s the rundown:

Shimano Dura-Ace Power Cranks Dual: $1,500

Specialized S-Works Power Cranks Dual: $1,150

Specialized S-Works Power Cranks Single: $750

Shimano Ultegra Power Cranks left-arm upgrade: $525

Shimano 105 Power Cranks left-arm upgrade: $425

(left-arm upgrades do not include the right crankarm)

Lightest, Most Accurate?

Specialized says that the complete weight of a dual-sided system, including the CR2032 batteries that power it, adds just 30 grams to the overall weight. The S-Works Power Cranks weigh in at a (claimed) 440 grams in a 172.5mm crankarm, without chainrings. That is certainly light, but in the fight over who has the lightest setup, remember that the power meter hardware itself is a very small part of the equation and, from power meter brand to brand, differences on identical crankarm configurations like Shimano Dura-Ace, for example, will be in just a handful of grams.

What’s more interesting is the accuracy claim. Specialized says that the Power Cranks are within +/- 1.5 percent accurate. That’s identical to what a number of other power meter makers, including 4iiii, Stages, and Quarq, claim for accuracy, and actually a wider range than what the industry’s oldest brand, SRM, claims (less than one percent). So how does Specialized justify its “most accurate” claim?

Power meter accuracy validation is typically performed via static torque measurement (loading the crankarm to measure deflection), or on an ergometer with a machine-driven crankarm spinning under a specified load at a specified wattage. But those setups, while highly controlled, don’t begin to reflect the complexity of how riders actually pedal. A human rider’s pedal stroke is never perfectly smooth; there are dead spots and uneven application of power throughout the stroke, and force is not always consistently applied in the same direction, owing to biomechanical quirks, or even changes in pedaling style at various intensities.

Rodger Kram, director of the University of Colorado's Locomotion Lab, explains the math behind the test to measure real-world accuracy. Photograph courtesy of joe lindsey

So Specialized went to a frequent collaborator, the Locomotion Lab at the University of Colorado, to devise a different protocol. The Locomotion Lab, run by renowned biomechanist Rodger Kram, has worked with Specialized on a number of its Body Geometry products, as well as with component maker Fi’zi:k. Most famously, Kram and his team also worked with Nike to develop the Vaporfly 4%, the shoe used in last year’s “Breaking 2” marathon attempt.

Wouter Hoogkamer, a postdoctoral researcher, led the Power Cranks testing. The test was centered on a converted treadmill, which eliminates the variable of aerodynamic drag. With precise measurements of factors like total rider and bike mass, and the exact angle of the treadmill incline, the researchers could calculate total mechanical power needed for the rider to maintain a given constant speed on that incline. The team accounted for rolling resistance during testing via a simple but effective counterweight system. Drivetrain frictional losses, the only force the team couldn’t directly measure, is factored into the equation, based on independent drivetrain tests conducted by Friction Facts. The powermeter readout in test runs was then measured against the calculated total mechanical power figure to gauge accuracy.

Dr. Kram adjusts a simple but effective counterweight system that balances out rolling resistance forces that would upset the power readout. Melissa Mazzo, a doctoral candidate in Neurophysiology, was one of the test subjects for the real-world validation, which used an oversize treadmill. Photograph courtesy of joe lindsey

The researchers tested units at a wide variety of temperature ranges by cooling and heating the crankarms and measuring the results against room-temperature tests, and test riders used a variety of gearing combinations and pedal cadences in both seated and out-of-saddle test runs across multiple power outputs. Through all of that, the team found the power meters were within 1.5 percent “real world” accuracy, with consistent results at all temperatures measured (measurement drift due to temperature changes is one of the big bugaboos affecting powermeter accuracy).

There are a few caveats there: the CU team has only tested prototype Power Cranks so far, not production versions. They have only tested a few models from competitors, and those models and brands were not disclosed. And they did not test the Power Cranks with a standard laboratory ergometer protocol as a benchmark. So the accuracy claim, at this point, needs further validation.

Test Impressions

It’s also worth noting that absolute accuracy isn’t the most proximate concern for many users. They’re more focused on aspects like consistency, durability and data integrity. We have spent very little time on the new Power Cranks, and that experience so far hasn’t given us any indication that the system is materially different than most of the other good power meters we’ve tried. We’ve experienced no issues with Power Cranks – no data dropouts or corrupt files, no pairing or antenna issues, and the power files we’ve seen so far seem to match what we get with other power meters. We’ll report back when we’ve had more time on it.

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